Cerebral hemodynamic impairment due to high-grade carotid artery stenosis can impair cognition even in the absence of stroke, contributing to cognitive decline either directly, or as a consequence of a higher occurrence of silent infarction. Although there is good preliminary evidence from case series and physiological studies that hemodynamic impairment affects cognition in patients with carotid occlusive disease, treatment of this condition has never been tested in a randomized clinical trial. We propose to conduct an ancillary study to the NINDS-sponsored CREST-2 trial, a pair of outcome-blinded, Phase 3 clinical trials for patients with asymptomatic high-grade carotid artery stenosis which will compare carotid endarterectomy plus optimal medical therapy (OMT) versus OMT alone (n=1,240), and carotid artery stenting plus OMT versus OMT alone (n=1,240) to prevent stroke and death. . Our application addresses the intriguing question of whether cognitive impairment can be reversed when it arises from abnormal cerebral hemodynamic perfusion in a hemodynamically impaired subset of the CREST-2 ?randomized patients. We will enroll 500 patients from CREST-2, all of whom receive cognitive assessments at baseline and yearly thereafter. We will identify 100 patients with hemodynamic impairment as measured by an inter- hemispheral MRI perfusion ?time to peak? (TTP) delay on the side of stenosis. Among those who are found to be hemodynamically impaired and have baseline cognitive impairment, the cognitive batteries at baseline and at 1 year will determine if those with flow failure who are assigned to revascularization arm in CREST-2 will have better cognitive outcomes than those in the medical- only arm compared with this treatment difference for those who have no flow failure. We hypothesize that hemodynamically significant ?asymptomatic? carotid disease may represent one of the few examples of treatable causes of cognitive impairment. If cognitive decline can be reversed in these patients, then we will have established a new indication for carotid revascularization independent of the risk of recurrent stroke.